1. Field of the Invention
This invention relates to cylinder locks and more particularly to cylinder locks which cannot be opened without a key by any of the known "lock-picking" techniques.
2. Description of the Prior Art
The inventor's U.S. Pat. No. 3,761,193, issued Oct. 2, 1973, discloses a cylinder lock having a plurality of pin assemblies selectively configured to engage a shoulder within the cylinder bore in the event that an attempt is made to pick the lock. Each pin assembly comprises a driver portion, a center portion, and a lower portion. The center and driver portions each contain unique transverse grooving which cooperates with the shoulder in the cylinder bore to prevent pin movement whenever rotational pressure is applied to the plug without first inserting the proper key.
This patent includes the basic components present in most cylinder locks: a key plug, a cylinder surrounding the key plug, a set of tumblers in the form of cylindrical pins mounted in radial bores in the key plug, and a set of driver pins mounted in radial bores in the cylinder corresponding to those in the key plug. When there is no key in the plug, the tumbler and driver bores are in alignment, and the driver pins project across the shear line between the cylinder and key plug into the tumbler bores in the plug, preventing it from being turned within the cylinder. In order to open such a lock, a key is inserted having indentations defining selective recesses and projections which cause each of the tumblers to be held to a definite lifted position such that the dividing line between the drivers and the tumblers in each bore coincides with the shear line between the cylinder and plug. When all pins are appropriately positioned, the plug is free to rotate.
There are two well-recognized techniques for picking conventional cylinder locks. In the first, the plug is forcibly turned relative to the cylinder, to the maximum extent allowed by the slight clearance between the pins and their respective bores. Whie maintaining torque upon the plug, each tumbler is carefully pushed upward so that the driver associated with it moves up into its bore in the cylinder until it comes to rest at the shear line due to the ledge created by the slightly rotated position of the plug. When all drivers have been pushed back into the cylinder bores in this manner, the plug can be freely turned and the lock opens.
In the second familiar technique, the tumblers and associated drivers are all initially pushed upward so that the tumblers enter the driver bores. The plug is then forcibly turned as far as it will go and the tumblers are allowed to drop under the action of the driver springs to their normal positions clear of the driver bores. This occurs because at the beginning of their downward movement, the tumblers are already partly within the plug. On the other hand, the drivers are not capable of following their tumblers because they will be stopped by the ledges that are created at the shear line due to the rotated position of the plug. Here again, the plug can be turned freely after all tumblers have dropped.
Another method of opening a lock for which one does not originally have a key, is the technique of "impressioning". In using this technique, a blank key is inserted into the plug and slight rotational pressure is applied during up and down movement of the key. When the key blank enters the lock it pushes all of the tumblers up into the driver bores and when plug pressure is applied, putting upper and lower bores out of register, these pins become trapped. Subsequently, when the key is raised, the pins being unable to move away, burnish small marks, or impressions, on the edge of the key. The key blank is withdrawn, the impressions lightly filed, and the key reinserted. During succeeding insertions, where filing has been done, the tumbler pins rest lower by a small amount. The process is repeated until one of the pins reaches it shear line. At this point, the pin is no longer trapped in a driver bore and becomes free-floating and incapable of making an impression on the key. This informs the impressionist that the pin is "open" and he simply continues with the procedure until all the remaining pins are open. The end result is a hand-filed key. A very undesirable feature of having a lock that can be breached by impressioning, is the fact that the lock can then be reopened at any future time leaving no clue of the opening.
In addition to the inventor's own work in this field, a large number of lock structures have been developed in an attempt to defeat the picking of locks by the above-described methods. The resulting structures, have included in various combinations the serration or grooving of the cylinder bores, the plug bores, the driver pins, and the tumbler pins. When transverse grooves are provided in the pins and bores, it will be understood that the pins cannot move freely except under prescribed conditions such as when their dividing lines are coincident with the shear line between the cylinder and the plug. This prevents a lock-picker from the simple application of the aforedescribed techniques. On the other hand, with a knowledge of the structure of such locks, a skilled lock-picker can generally "feel" or sense the relative positions of the driver and tumbler pins and given sufficient time will open all of these locks.
A series of locks that are quite effective in their ability to prevent illegal entry, are the Spain locks disclosed in U.S. Pat. Nos. 3,499,302, 3,499,303 and 3,722,240. These locks, in addition to other features, include tumblers in the key plug which must be positioned by a properly bitted key reciprocally to clear the shear line and also rotationally to allow a fence member to be cammed out of engagement with the cylinder shell. Only when the appropriate combination of translational and rotational position of each tumbler is effected, may the key plug be rotated.